1. Field of the Invention
The present invention relates to a process for preparing phosphites and their transition metal complexes and also to their use in catalytic processes.
2. Brief Description of the Prior Art
Phosphites have achieved great importance, especially in homogeneous catalysis, since they are capable of complexing to a transition metal, controlling its catalytic activity and in some cases transferring stereochemical information to a substrate.
It is known that stereoisomerically enriched phosphites which are derived from 2,2′-binapthols in particular are suitable for asymmetric hydrogenations (see also DE 100 27 505 A1 and Reetz, Mehler, Angew Chem., Int. Ed. Engl. 2000, 39, 21, 3889). The stereoisomerically enriched phosphites are prepared by initially reacting the corresponding stereoisomerically enriched 2,2′-binaphthols with phosphorus trichloride to give binaphthylphosphorus chlorides and subsequently reacting these with monoalcohols.
Alternatively, the conversion can be effected at −78° C. using a monoalkoxide (see also P. H. Dussault, K. R. Woller, J. Org. Chem. 1997, 62, 1556–1559).
A disadvantage of this process is that in the preparation of binaphthylphosphorus chlorides, one has to use very low temperatures of −78° C., which can only be realized on the industrial scale at great cost and inconvenience.
Illustratively, EP-A 729 965 discloses a process for preparing biphenyl phosphites in which phosphorus trichloride is additionally reacted with a monoalcohol and subsequently with a biphenol in a one-pot reaction. A disadvantage of this reaction is that, in the second step, an amine is required as a base, which forms a hydrochloride soluble in the reaction medium. Also, the operation has to be effected in high concentration and in aromatic hydrocarbons as solvents, in order to achieve high yields. However, such specific conditions are unacceptable for industrial use, because broad application to differently substituted phosphites is hardly possible.
J. M. Brunel, G. Buono, J. Org. Chem. 1993, 58, 7313–7314 disclose a process for optical resolution of 1,1′-binaphthyl-2,2′-diol which proceeds via the preparation of diastereomeric binaphthylmenthyl phosphites by initially reacting phosphorus trichloride with one equivalent of L-menthol and subsequently adding the racemic 1,1′-binaphthyl-2,2′-diol. A disadvantage of this process is that the intermediate is not isolated and is therefore not purified.
There was therefore a need to provide a process for preparing phosphites and in particular stereoisomerically enriched phosphites which ensures high yields and high product purities.